Custom Document Sets opening as Folders

Question

I have a custom library with custom content types derived from document set. The library doesn't recognize them as document sets, and when clicked on they open as a folder instead of to the welcome page. An OOTB Document Set opens correctly though. How do I fix this?

Answer 1

It seems ProgId="SharePoint.DocumentSet" needs to be added to the content type definition as a property within the ContentType declaration to identify it as a DocumentSet. However adding it to the content type didn't seem to be enough, I had to add it in the library Schema file as well, like so:

<ContentType ID="*some GUID*" Name="*name of content type*" ProgId="SharePoint.DocumentSet">